The growth of a coral reef

Coral grows in different ways: coral can divide and therefore become larger, but it can of course also reproduce, this is done through sexual and asexual reproduction. The growth of a coral reef is going very slowly and that is why it is so dangerous what people are currently working on: destroying coral.

The growth

The shape of a coral colony is influenced by the way in which a polyp forms buds to make copies of itself. With many species, the polyp gradually becomes oval and forms a second mouth opening, after which it splits into two separate polyps. In other species, the polyps do not divide completely and long ribbons of partially populated polyps are formed, which form wave patterns on the surface of the colony, as is the case with different types of brain corals. The enormous variation in shape and size of coral colonies depends not only on the species, but also on the place where the coral grows. On the forefront, with a strong wave, corals tend to flatten and kiss-like shapes. In a sheltered environment, such as the reef lagoon, many branched forms occur.
The available amount of light also influences the shape of a colony. Some species adapt easily and differ in shape because they live closer or farther from the surface. Some species seem to spread further at depth so that their zooxanthellae can catch as much light as possible. There are species of coral that easily adapt to a new environment by, for example, using a large surface, but there are also species that cannot live anywhere if it does not meet their 'requirements'. After the death of coral they leave hard skeletons on which new species settle or where other organisms will live. The dead coral at the foot of the reef falls apart and forms sediment and coral sand, which over time is compressed into coral lime.
Most corals grow very slowly and therefore the formation of a reef takes centuries. The massive corals grow the slowest, about 5-25 mm per year. These are the corals that survive the most. Researchers have now discovered that corals also have annual rings, just like trees. These annual rings are visible with the help of X-rays. A number of corals have been researched on the Great Barrier Reef in Australia and corals are found that are between 800 and 1000 years old. Branched corals grow much faster, this is very different per species. They also increase the amount of calcium carbonate in the water.
Reefs grow less rapidly than the individual colonies, because the reef is damaged every year by storms and waves. There are also hunting and grazing animals that attack the reef. When a coral dies, it is quickly overgrown with seaweed. Then it is attacked by all kinds of animals; dead corals die in this way within a year. After these drilling animals, the eroding organisms make their mark. Worms are the first to penetrate, they drill tunnels and create a habitat for the animals that come after them. Sponges have the most destructive effect. There are species that drill into the coral by separating chemicals that break down the lime skeleton into small flakes. These fragments sink slowly and become part of the structure of the reef. Grazing animals also influence the growth of a coral reef. Doctor fish, parrotfish and sea urchins eat seaweeds that grow on dead or living coral and on crustaceans. Parrot fish grind limestone, which they ingest with the algae, and excrete it again as fine sediment. Sea urchins in turn remove large amounts of dead coral when they graze on wire algae. Because of all these activities you cannot actually determine how fast a reef grows. Researchers are currently trying to calculate it by determining the amount of calcium carbonate that is produced. Biologists estimate that a reef is between 1-15 mm per year higher. The speed does depend on the type of reef: barrier reefs and coastal reefs grow faster than atolls. The average reef growth is 7-10 mm per year.

The reproduction

Corals reproduce in two ways: sexual and asexual.
In asexual reproduction, a parent polyp produces a daughter polyp by budding, which is genetically an exact copy of the parent, to which they remain connected through living tissue. Living polyps only occur on the outside of the colony: the inner part is an accumulation of dead skeletons from earlier polyps. New colonies can arise from fragments that have been broken off from a larger colony, provided the circumstances are good enough for this fragment. There are also coral species that live in solidarity: their polyps grow large, but do not form groups. Corals can reproduce not only sexually, but also sexually. They then release eggs and sperm into the sea. There the eggs are fertilized by the sperm and new individuals, the planula larvae, develop.
Every year a mass pairing takes place among the corals. Every year 130 species spawn on the same night, thereby releasing millions of eggs and sperm particles at the same time. This is therefore a unique form of synchronization in the animal kingdom which is known under the term 'spawning'. This night is more or less predictable by the biologists. It is a night after full moon in late spring or early summer.
Temperature, tide, day length and moon phase play a role in this event. The swirling mass of eggs and sperm confuse predators and that ensures that the fertilized eggs have a higher chance of survival. Due to the low tide during the equinox, the time at which plowing is usually done, eggs and sperm would mix better before being washed away. Not only does coral make synchronous pairs, but also starfish, marine articulated worms and other invertebrates on the reef spawn synchronously. After the eggs have been fertilized, the coral larvae float up to the light and mix under the plankton. They are currently 1 mm long and still have all kinds of different shapes, depending on the species. The parent polyp has given them zooxanthellae in their tissue cells. If they stay alive, they float in the water for a few days or weeks. Eventually the larva swims down and fixes itself on a hard, stony surface, such as a dead coral skeleton or a bare rock. There he turns into a polyp and starts to form a lime skeleton. He will reproduce asexually and in this way a new colony will be created.

Video: A Coral Reef Revival. The Atlantic (February 2020).

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